A program is proposed to investigate the feasibility of a new ultra-small wireless in vivo dosimeter concept that could have the potential to enable entirely new approaches to radiation cancer therapy. The enabling technology is a radiation-sensitive variable capacitor (varactor) based upon fully-depleted silicon-on-insulator (FDSOI) semiconductor technology, a platform that provides an ideal mechanism for transducing radiation-generated charge into an electromagnetic frequency shift that can be detected using wireless telemetry. For radiation therapy, verification of both the delivered dose and the location where it is delivered is critical for successful treatment. (This is particularly important for cliical situations where delivered dose is not well understood or modeled. These include cervix cancers; targets that move significantly during a treatment session, such as those located in the lung; and tumors located near sensitive structures such the vertebral column or the optic nerve. The detectors could also be extremely useful in proton therapy.) Wireless dosimeters for in vivo use have been developed previously, but these dosimeters are too large for routine use. The extremely small sizes of the FDSOI dosimeters (fully packaged parts could have volumes on the order of 1 mm3) could allow both dose and position information to be obtained from precise locations using a minimally invasive procedure. In addition, the small size and ability to tune to the devices to different resonant frequencies allows multiple sensors to be placed. In order to realize this goal, a research program has been developed to assess the feasibility of this concept. A design study for FDSOI varactors is proposed to determine the structural parameters that will allow the best trade-off between capacitance tuning range and quality factor, the two main parameters that ultimately affect the sensitivity of the dosimeter. The devices will be fabricated at the University of Minnesota and their basic characteristics will be measured. Finally, the devices will be characterized under clinically relevant irradiation conditions. The research team consists of an electrical engineering professor, a radiation therapy medical physicist, (and an oncologist clinician) who have precisely the right mix of expertise to design and fabricate the devices and to evaluate their suitability to meet the requirements for use in cancer therapy.